In the field of radio communication equipment, MIMO (Multi-Input Multi-Output) communication, which uses array antennas, can increase the communication speed without expanding the frequency band in use, and improve the overall system throughput (see, for example, Non-Patent Document 1).
Recently, in the field of wireless LAN, introduction of MIMO communication technology is being studied to standardize IEEE 802.11n, This technology is mentioned in the draft of the standard by IEEE in 2007. Similarly, MIMO communication technology is also being studied to increase the data transmission speed of mobile telephones, mobile wireless data terminals and so on.
In a conventional wireless communication system not adopting MIMO communication technology, communication quality is determined by the electric field intensity in the receiving point. By contrast with this, in a communication system adopting MIMO communication technology, communication quality is determined not only by the electric field intensity in the receiving point but also by the state of the radio propagation communication channel between the transmitting side and the receiving side. Therefore, the MIMO communication system needs to monitor the state of the radio propagation communication channel (this is referred to as “channel estimation technique” and see, for example, Non-Patent Document 1, Chapter 2 to 3, “channel estimation/equalization technique”), and select optimal received parameters based on the monitored state of the radio propagation communication channel.
Particularly, in the MIMO communication system having a portable personal computer (PC) adopting MIMO communication technology, the location and environment in which radio equipment is placed and used change frequently. The changes in the location and environment, in which radio equipment is placed and used, influence the state of the radio propagation communication channel in the MIMO communication system. Therefore, when there is a specific relationship between an arrangement of antennas in radio equipment and propagation environment of the surroundings of the radio equipment, there are cases where the state of the radio propagation communication channel deteriorates. That is, there are cases where quality of MIMO communication deteriorates or the system throughput decreases.
Therefore, conventionally, there is a MIMO communication system as disclosed in, for example, Patent Document 1. In this MIMO communication system, the receiving station has: a state index calculating section that calculates a state index that represents the current communication state, using all or part of transfer functions; and a communication state display section that changes the content to display according to the value of the state index. Further, this receiving station has an external interface section that transfers the state index to an external terminal connected with the receiving station via wired or wireless connection. FIG. 1 shows a conventional MIMO communication apparatus disclosed in Patent Document 1.
The communication state index calculating circuit in FIG. 1 performs numerical calculation to calculate the state of the radio propagation communication channel as an index. The display section presents a display matching the resulting state index or a state index obtained by combining state indices calculated according to a plurality of methods. Then, by referring to the index displayed on the display section, the user can change the location to place the MIMO communication apparatus or control diversity in the MIMO communication system.
Further, the MIMO communication system disclosed in Patent Document 1 assumes a method generally referred to as “Zero-Forcing (ZF) method,” as the method of detecting signals transmitted based on MIMO technology. The above communication state index calculating circuit decides the value of the determinant of the channel estimation matrix, so that it is possible to change the location to place the MIMO communication apparatus and controls diversity in the MIMO communication system.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-211566    Non-patent Document 1: JPO Documents, Standard Technologies, Electronics, 2004, “Related technology of MIMO(Multi Input Multi Output),”http://www.jpo.go.jp/cgi/link.cgi?url =/shiryou/s_sonota/hyoujun_gijutsu.htm